Japanese Patent Publication No. 3382061 discloses a driving unit for driving a resonance system comprising an elastic body and a moving element supported by the elastic body. The driving unit is used in an electric shaver as a linear actuator which reciprocates an inner cutter.
The driving unit is energy efficient because the moving element reciprocates as the resonance system conserves kinetic energy of the moving element and elastic energy of the elastic body and converts them to each other alternately. In actuality, since the energy is consumed by a load and so on, the driving unit has to give consumed energy to the moving element in order to keep the reciprocating motion.
So, in this driving unit, as shown in FIG. 24, a controller of the driving unit gives an alternating voltage of a rectangular wave to an electromagnet every half-cycle, and reciprocates the moving element with a constant amplitude by controlling an voltage application period (Ton) and a phase (Tph) of the alternating voltage.
Explaining in more detail, when the rectangular voltage is applied to the electromagnet, a current in the form of a triangular wave, as shown in FIG. 24, flows through a coil of the electromagnet. Vibration force which moves the moving element will increase or decrease in response to the amount of the current flowing through the coil. For example, when the amount of the current flowing through the coil increases as shown by a dashed line in FIG. 24, the vibration force will increase, and, on the other hand, when the amount of the current flowing through the coil decreases, the vibration force will decrease. So, in this driving unit, the controller detects a motion of the moving element every half-cycle, and if the width of the moving element is larger than a target width, the controller decreases the voltage application period, and if the width of the moving element is shorter than the target width, the controller increases the voltage application period. Furthermore, the controller applies the voltage to the coil when the moving element goes a predetermined phase (Tph) from a top dead center or a bottom dead center so as to apply the voltage at the right time in keeping with the moving direction of the moving element.
As mentioned above, the conventional driving unit controls the moving unit by varying the voltage application period (Ton) and the phase (Tph) of the voltage in order to make the moving element do an intended motion, such as a reciprocating motion with constant amplitude. However, although the conventional control method can make the moving element do an intended motion, it does not take into consideration an influence of a waveform of the current flowing through the coil on energy efficiency. Therefore, in the conventional driving unit, a current in the form of the triangular wave having many harmonic components flows through the coil, as mentioned above, so that a momentary current becomes very high at the conclusion of the energization. As a result, an energy loss due to resistances of the coil, control circuit, etc. increases, by which heating values of electronic components and coil increase, and total energy efficiency decreases.